Information storage systems that use an optical beam to sense certain physical conditions on the surface of a storage medium representing bits of stored data are well known to have very high density information storage capacities. In order to take useful advantage of these high storage capacities, it is desirable that these systems be provided with head positioning drives, i.e. tracking actuators, that are capable of very fast seek times to position the focused optical beam on a desired data track selected by the host computer out of many thousands of such tracks. In this seek mode of operation, the head mass must be moved rapidly through optimally determined acceleration and deceleration profiles between beginning and ending track locations that could be adjacent to each other or several thousand tracks apart. Additionally, once the desired track has been reached, the head must be precisely controlled in a tracking mode of operation during which the beam is maintained in centered registry with the track, which is typically less than a micron in width, as data is being read or written. It is an object of the present invention to provide a tracking actuator for an optical or magneto-optical system that is not only capable of providing the level of tracking mode performance required by optical drives but also provides track seek (access) time performance that is at least comparable to, and in some cases better than, existing high capacity magnetic drives such as conventionally known Winchester and Bernoulli drives.
Various forms of tracking actuators are known to provide reasonably fast access time and good tracking control in magnetic disk drives of the aforementioned type. In a typical arrangement, the carriage for the read/write heads is mounted with ball bearing rollers on a pair of tracks, or races, and the carriage is driven by a stepper motor or voice coil motor to translate the head linearly in a radial direction over the data tracks formed on the surface of the recording disk. This arrangement has the advantage of providing a true linear tracking axis but has the disadvantage of imposing roller bearing irregularities that are not compatible with the extremely small, precise motions that are encountered in optical drives. U.S. Pat. Re. No. 32,285 is an example of a voice coil driven actuator utilized in a roller/race carriage type of hard disk drive system that employs a pair of separate voice coil motors positioned symmetrically on opposite sides of the carriage. Although not expressly mentioned in the patent, it is typical for the rollers to be mounted with ball bearings to eliminate friction. While satisfactory for magnetic drives, ball bearings suffer from fretting corrosion when subjected to the sort of micro-fine vibratory motion encountered in optical drives. Fretting corrosion occurs when the vibratory motion drives lubricant out from between the ball and race leaving a metal-to-metal contact during the period of vibratory motion.
The specification of the patent suggests in the introductory portion that s single drive coil somehow wrapped around the carriage could also be employed as an alternative to the pair of coils, however no embodiment of this type is disclosed, nor is it readily apparent from the disclosure how such an embodiment would be constructed. However, taking into account the pair of coils that are disclosed, it is apparent that a substantial amount of unused wire, i.e. portions of the coil not involved in developing the required motive forces, would be required to span the carriage space between the dual magnetic circuits on either side of the carriage. This unused wire is detrimental since it represents dead weight in the moving mass that undesirably increases the power consumption needed to achieve desired operating performance.
Moreover, purely magnetic information storage systems generally have tracking requirements that are not as precise or stringent as those found in the case of optical and magneto-optical storage system. Because of the extremely small track-to-track spacing found on optical and magneto-optical disks, on the order of 1.6 microns, information storage systems that use optical beams to scan recorded information on such narrow data tracks conventionally employ a two stage positioning arrangement. The first stage, or seek mode of operation, may typically involve a first drive apparatus, either stepper motor or voice coil, to move the entire mass of the head to rapidly position the beam in the vicinity of the desired track in as short an access time as is possible. In the second stage, or tracking mode of operation, a separate drive apparatus, usually another voice coil moved into coarse position over the tracks by the first actuator stage, is used to control the lateral positioning of just the beam objective lens to complete the positioning of the beam and to maintain the beam centered on the data track during read and/or write operation. In general, the reason for employing a separate tracking actuator for the objective lens, is the difficulty that conventional head positioning means have in achieving the required high frequency response for proper tracking error correction, as opposed to the seek mode of operation, particularly in compact, cost sensitive applications. U.S. Pat. No. 4,607,913 discloses a linear tracking actuator of the general type disclosed in the aforementioned U.S. Pat. Re. No. 32,285 but adapted for use with an optical read/write system. In this disclosure, an axially movable beam objective lens is mounted by flexures to a carriage between a pair of sleeves riding on guide rods. The sleeves are made of tetra-flouroethylene to reduce friction. A pair of drive coils are wrapped around the sleeves and have their outer turn segments disposed in longitudinal air gaps formed by elongated magnets in parallel with the associated guide rods. The objective lens for this system is carried in a separate mount spanning the parallel guide rails and is attached to the main carriage by the elongated flexures. A separate pair of radial drive coils are provided for the objective lens to provide additional drive force in proportion to the ratio of masses of the objective mount to the main carriage so that the flexures do not have to be relied on to convey the radial driving force from the carriage to the objective mount. This complex arrangement requires, among other things, that the magnets be extended to cover two sets of actuator coils.
In an effort to address problems of the nature described above, an early form of erasable magneto-optical disk drive was developed at Philips Research Laboratories in Hamburg, West Germany and which was reported in the Dec. 29, 1982 issue of "Electronics" magazine at page 45. In this arrangement, a Faraday effect optical light source and sensor assembly was mounted on the end of a bifurcated arm extending over a magneto-optical disk from a rectangular frame suspended by a pair of parallel flexure springs for friction-free movement of the arm and head along a tracking axis over the surface of the disk. This apparatus was a single stage actuator in that both the track seek and tracking modes of operation were accomplished with a single drive. The driving force for this apparatus was provided by a voice coil motor arrangement with the voice coil mounted on the frame spaced away from the light source head and with the plane of the coil turns oriented parallel to the tracking axis.
Although both of this voice coil, flexure mounted design proved to be operative for its intended purpose of eliminating the problems associated with track mounted designs, it did not prove to be satisfactory for commercial application. This actuator employed a beam source and detector cantilevered out beyond the flexure mount and required a separate, relatively large focus actuator to carry the tracking actuator flexure mount which added undesirable mass to the system. As such, this drive proved unsatisfactory to meet stringent performance requirements associated with competitive high capacity drive systems, and was not suitable for compact, low cost personal computer applications.
It is therefore an object of the present invention to provide a tracking actuator drive system for an optical or magneto-optical disk information storage system that avoids the drawbacks and disadvantages of prior actuator drive systems.
It is a further object to provide a tracking actuator drive system that lends itself to compact design for use in a half height format useful in personal computers.
It is yet a further object to provide a tracking actuator drive system that provides rapid seek times comparable to seek times of existing high performance Bernoulli and Winchester magnetic disk drives.
It is still a further an object to provide a single stage tracking actuator for an optical or magneto-optical disk drive system with relatively low mass that does not require a separate tracking actuator for the objective lens to maintain the optical sensing beam centered on the data track during read/write operation.